The Emperor of whe Wandenreich has shown not to let any occasion to execute a subordinate slip. He did it (or had Haschwalth do it) in every single fucking instance he could even remotely justify it, even though the justification would always be feeble at best. Therefore I've begun to think that he does it more than just on a whim.
My hypothesis is that His Yhwachness feels fully entitled to the powers he bestowed upon the Quincies, and thus is very quick to want to reclaim it if said Quincies fail to live up to his expectations. Because yeah, apparently being the source of all Quincies' powers, he reclaims them upon the death of said Quincies. He did it with the mixed bloods, why can't he do it with the purebloods?
A stands for "The Absorb" (name fully adjusted for Kubo's grammar). Yhwach can freely manipulate the bulk of the Quincy powers which he considers his sole possession. Him redistributing the Quincy power bulk among others is considered by him to be just an act of goodwill that can be undone at any moment. He only needs his subordinates to do his job for him, but in the long term it is actually in his best interests that they all die so he can have his powers back.
While he might have had genuine attachment to some of his subordinates (as shown by Royd when Yamamoto brought back those Quincies' ashes), the times were probably different and his approach was also different. Right now he's about the most heartless, egotistical prick ever, plus his motivations for this entire war are unsatisfactory. Just revenge on the Shinigami? Yeah, right.